The present invention comprises a reaction wheel assembly used to provide reaction torque in momentum stabilization of vehicles. In particular, the present invention comprises an improved reaction wheel assembly in which a rotor safing mechanism is employed. In the present invention, the rotor safing mechanism is used to isolate the rotor bearings from deflective forces that are incident upon the reaction wheel assembly during periods of high vehicular stress. In the case of a satellite, for example, the present invention is used in safing the rotor mechanism during launch of the satellite, a time during which damaging forces are brought to bear upon the relatively delicate components of the reaction wheel assembly.
During launch of a satellite, forces (or "loads") are generated by the reaction forces of a high inertia rotor structure exposed to vibration environments during payload launch and ascent. Effective bearing isolation using the present invention facilitates the use of smaller bearing systems. Further, savings in power consumption may be brought about by use of the present invention. Further, the present invention provides advantages as to size, cost and mass of the components in a reaction wheel assembly.
Reaction wheel assemblies of the type disclosed in the present invention include a rotor which acts like an axle positioned between two support points. The support points in the present invention are comprised of bearings, and the rotor is allowed to spin within bearings located at either end of the rotor.
Momentum stabilization is achieved in most reaction wheel assemblies by the use of a rotor web structure, which proceeds laterally and perpendicular to the rotor itself, with a relatively large mass at the distal end of the rotor assembly. During operation of reaction wheel assemblies, the forces acting upon the bearings are generated by bearing preload forces, misalignment forces, and residual rotor imbalances. Further, some undesirable forces that occur during operation may be caused by the housing precessional rate.
In the design of reaction wheel assemblies, the mass of the structural components is a critical factor. Since reaction wheel assemblies typically are used in conjunction with spacecraft, satellites, aircraft, or other vehicles that are in motion, the mass of the assembly is very important. Of course, the performance of a flight vehicle is greatly affected by gravity, and as in all space or satellite components, the weight of the component must be kept to an absolute minimum, while still maintaining the integrity and strength to carry out the mission.
There are several components of reaction wheel assemblies that must be minimized in terms of total mass and size to provide the most efficient operation, using the least amount of mass. For example, the mass of a reaction wheel assembly may be reduced by using smaller bearings at either end of the rotor. One of the advantages of using smaller bearings is that the viscous drag is considerably reduced, and correspondingly, the total power consumption of the reaction wheel assembly is reduced. Power consumption is a critical factor in satellite or spacecraft systems because the majority of the power consumed and used on a spacecraft or satellite must be carried into orbit in the form of batteries or other energy storage devices, and these sources of power are quite heavy. Thus, the less power consumed by the reaction wheel assembly, the greater its efficiency for its intended purpose.
If smaller bearings are used in a reaction wheel assembly, the forces brought to bear upon the bearings during launch may exceed the operating capability of the bearings. During launch, resultant forces are applied to the rotor web assembly, and a large moment of force is applied to the bearings from the intense vibrational environment of launch conditions. Further, during launch, harmonic forces are sometimes generated by the natural frequencies which may add upon themselves, producing a harmonic effect that may provide extremely high forces to the bearings within the reaction wheel assembly.
If the bearings of the reaction wheel assembly are subjected to forces that are too great, they will fail, causing failure of the entire reaction wheel assembly. Of course, without the reaction wheel assembly, a satellite, spacecraft, or other vehicle may lose its ability to direct itself along the correct flight path, resulting in a disastrous failure of the mission.
The problem is one of isolating or "safing" the rotor assembly mass from bearing loads that occur during periods of high vehicular stress, such as during launch of a satellite. Several different approaches have been proposed in an attempt to solve the problem, but until now, none of these approaches provided a satisfactory solution.
At least two prior methods have been used to restrain the rotor web of a reaction wheel assembly. The first method is a captivating mechanical rotor method, in which the outer rim of the rotor web assembly (which contains the large amount of mass necessary for the rotor function) is prevented from deflection by physical connection (or tying) the rotor to the housing. The primary disadvantage of the captivating mechanical restraint method is weight inefficiency and mechanical complexity while also requiring pyro release techniques for subsequent rotor operation. For example, the design must facilitate the release of the rotor web during operation of the reaction wheel assembly while still adequately securing the rotor web assembly during launch. The release mechanism is clumsy, and it is an inefficient method of containing the rotor web.
Other prior art methods attempt to solve the problem by adding mass to the rotor itself, thereby providing a dynamic response of the rotor to the forces brought to bear upon the rotor. In such applications, it is said that the rotor is "tuned" by changing the natural frequency of the rotor to a level such that vibrational tendencies are kept to a minimum.
For example, the addition of mass to the rotor may change the natural frequency of the rotor, thus helping to "dampen" the frequency response of the rotor, by counteracting the gain that is seen during periods of stress, such as during a launch. The drawback to this prior art method is that the rotor may only be tuned to specific environmental frequencies, and if a stress environment provides a frequency for which the rotor cannot be correctly tuned, dampening of vibrational forces will not occur. Further, there is a relatively narrow bandwidth for these types of rotor safing assemblies. Another drawback of this prior art method is that applying mass to the rotor itself is known to cause rotational problems and instability of the rotor due to inferior balance characteristics of these systems.
The present invention, on the other hand, solves the problem of providing a safing mechanism that will adequately secure a reaction wheel assembly in a high stress environment, and then facilitate a release of the assembly to allow free movement of the rotor during operation of the reaction wheel assembly. In the present invention, the rotor bearing assembly is safely stowed by setting the rotor structure into a preloaded configuration during the time period of high vehicular stress, such as during the launch and ascent phase of a satellite mission.
The preloaded configuration of the present invention restrains deflection of the rotor structure thereby minimizing or eliminating vibrational transmission from the rotor structure to the delicate bearing assembly. All of these benefits may be accomplished while using a smaller bearing structure comprising less mass, at reduced cost, and in a smaller size than bearing systems used in the prior art.